In general, hydraulic drive machines, such as construction equipment, are configured so that a drive command signal, which specifies a control input for a plurality of operating levers, is applied to a plurality of corresponding operating valves (flow control valves). The above drive command signal changes the areas of the apertures of these plurality of operating valves, thereby driving a corresponding plurality of hydraulic actuators. In other words, when a plurality of operating levers are operated simultaneously, pressure oil discharged by a hydraulic pump is supplied to a plurality of hydraulic actuators via a plurality of operating valves on a plurality of pressure oil supply channels, and these plurality of hydraulic actuators are driven simultaneously.
With this configuration, there is something called a load sensing system, which serves as the technology that cancels the so-called load dependency of the drive velocity of hydraulic actuators operated in combination.
With this system, a valve, called a pressure compensation valve, is provided either between a hydraulic pump and a flow control valve, or between a flow control valve and a hydraulic actuator, and works to compensate for the differential pressure of pressure across a valve for pressure oil, which flows through a flow control valve, so that this differential pressure is the same value for all drive shafts (In construction equipment, a drive shaft refers to a boom, arm, et cetera.). That is, in the general formula for a hydraulic circuit, EQU Q=c.multidot.A.multidot.(.DELTA.P)
(provided that Q is the flow that passes through a restrictor of a flow control valve, c is the capacity constant, A is the area of a restrictor aperture, and .DELTA.P is the differential pressure across a restrictor), the load sensing system works to achieve a flow Q, which is proportional to an operator-ordered drive command value (aperture area A) by making the differential pressure .DELTA.P the same for each drive shaft.
Further, the load sensing system works to control the discharge pressure of the hydraulic pump so that the discharge pressure of the hydraulic pump achieves a pressure whereby the above across-valve differential pressure is added to the maximum value of the load of the operating hydraulic actuators. This prevents changes in velocity (load dependency) resulting from differences in the load of each hydraulic actuator when operated in combination.
However, there are drawbacks to this system, such as complex valve configurations, and susceptibility to hunting as a result of poor hydraulic stability.
Accordingly, to solve for this problem, Japanese Patent Publication No. 6-41762 and Japanese Patent Publication No. 6-41764 propose the configuration of a system that does not utilize the above-mentioned pressure compensation valve.
That is, the points disclosed in the above-mentioned announcements make use of the above-mentioned general formula for a hydraulic circuit, EQU Q=c.multidot.A.multidot.(.DELTA.P)
and strive to find, by reverse calculations from the relational expression EQU A=Q/(c.multidot.(.DELTA.P)),
the aperture area A for achieving the target flow Q when there is a differential pressure .DELTA.P.
For arbitrary differential pressures .DELTA.P, which differ like this for each hydraulic actuator, the dependency of actuator velocity during combined operation is canceled by reverse calculating from the above general formula the aperture area required to achieve the various target flows.
Further, another method for canceling the above dependency without using a pressure compensation valve is disclosed in Japanese Patent Application Laid-Open No. 4-351304.
As disclosed in this announcement, the square root of the ratio between a differential pressure set in advance and the detected value of the differential pressure across a flow control valve is used as the correction factor to compensate for the drive command value (control input from an operating lever) for pertinent flow control valves of drive shafts other than the drive shaft with a minimum differential pressure across the flow control valve. This compensates for the drive command value so that the valve opening (aperture area) becomes as small as a drive shaft with a large across-valve differential pressure (drive shaft with a small load).
As disclosed in the above-mentioned Japanese Patent Publication No. 6-41762 and Japanese Patent Publication No. 6-41764, when finding the aperture area A, it is necessary to divide by the across-valve differential pressure .DELTA.P.
However, as a result of hydraulic pump flow saturation, which proves troublesome during the combined operation of hydraulic actuators, the differential pressure .DELTA.P between the hydraulic pump and a hydraulic actuator can often become a value in the vicinity of 0 (kg/cm.sup.2). Under these circumstances, there are times when the above-mentioned division operation is not possible.
Further, because processing requires division by a number approaching zero, the detection error of the pressure detector for detecting a differential pressure that approaches this zero impacts greatly on the accuracy of the arithmetic process, and control. Thus, to ensure better-than-constant accuracy requires a high-precision pressure detector. This is a problem in that it increases costs.
Furthermore, because the system determines whether or not it is possible to distribute the flow targeted for each hydraulic actuator from the dischargeable output of the hydraulic pump, complex processing is also required to correct target flows to each hydraulic actuator when the above-mentioned hydraulic pump saturation occurs.
Conversely, as disclosed in Japanese Patent Application Laid-Open No. 4-351304, similar to the above, when an across-valve differential pressure .DELTA.P is corrected, since this corrected differential pressure becomes the denominator, to avoid having zero (near zero) as the denominator, the system is designed to perform control so that this correction processing is not carried out for a drive shaft with a minimum across-valve differential pressure .DELTA.P.
However, as a result of this, there is the problem that when switchover to the drive shaft with the minimum across-valve differential pressure .DELTA.P takes place, it is necessary to switch from correction-based control to ordinary control. In that instant when switchover occurs, the continuity of drive command values to hydraulic actuators is interrupted, generating a shock each time switchover occurs.
Furthermore, as disclosed in this announcement, with regard to hydraulic pump control, what is called pump load sensing control is essential so that the differential pressure between the discharge pressure of the hydraulic pump and the maximum load pressure of the plurality of hydraulic actuators (minimum differential pressure) becomes the differential pressure that was set in advance. The idea is to digitize a conventional load sensing system as-is, and to deal with a small differential pressure by shutting off control.
The invention of the present application takes these facts into consideration, and has as a first object to make do with a simple hydraulic circuit, which does not utilize a pressure compensation valve, and to enable the use of an inexpensive, low precision pressure detector, to enable the maintenance of continuous control at all times using simple controls, and to get by without shocking the operator or the mechanical parts, and to cancel the load dependency of a hydraulic actuator flow during combined operation without limiting the control system of the hydraulic pump.
Further, the hydraulic actuator pressure compensation characteristics of all of the above-described prior art are univocally established, and these pressure compensation characteristics cannot be changed according to circumstances. As a result, prior art is unable to satisfy the following requirements.
That is, if we consider work that requires pressure compensation, and work that does not require pressure compensation, for work that requires fine lever control capabilities, such as suspension work, normal surface adjustment work, and finishing work, because the load dependency of the flow (actuator velocity) greatly impacts work efficiency, pressure compensation is required often.
Conversely, when performing release work following excavation, and when moving the cutting edge of the bucket to the next excavation site after dumping the arm, an operator prefers "load-bearing" movement during rough, full-lever operations.
When pressure compensation is constantly applied even during full-lever operations such as these, if the operating lever of a high load shaft is operated even slightly, the discharge pressure of the hydraulic pump rises abruptly at that moment, the dischargeable flow determined by the equivalent horsepower performance of the hydraulic pump drops, and the flow to other drive shafts also increases. This gives rise to the problem of the velocity of the hydraulic actuator on the light load side decreasing more than necessary.
During this kind of full-lever operation, the speed of the light load working machine is required more than split control in accordance with the control input of the operating lever. What is required is believed to be a "load-bearing" split, that is, control that weakens pressure compensation.
Further, an operation performed frequently during hydraulic excavator work is "rough-combing" whereby the surface of the ground is leveled. During this operation as well, an operator prefers to move the cutting edge of the bucket roughly horizontally over the surface of the ground using a full-lever boom-up, arm-excavation operation. If there is no pressure compensation at this point, because there is little pressure for raising the boom, the cutting edge of the bucket moves roughly horizontally over the ground. But when all-out pressure compensation is applied, it gives rise to the problem of the trajectory of the cutting edge of the bucket rising high into a circular arc.
In other words, when the pressure compensation function is put to use univocally, during fine control operations, combined operation can be readily performed using the operating levers without need for concern. However, there is a problem during combined full-lever operations in that speedy, "load-bearing" work cannot be carried out using conventional rough operations.
The present invention takes this fact into consideration, and has as a second object enhancing lever controllability, and improving work efficiency by making it possible to change pressure compensation characteristics of a hydraulic actuator in accordance with operating lever operating status and load pressure.